The three major goals of this proposal are: 1) to determine the regions of the infuenza virus (A/PR/8/34) hemagglutinin (HA) molecule which serve as target structures for anti-viral B- and T-cells. With respect to the HA-structures seen by antibodies, the previously constructed antigenic map will be refined and expanded by antibody-mediated selection and antigenic and structural characterization of some additional PR8 virus mutants and by generation and subsequent mapping of anti-HA2 antibodies. The HA-structures involved in stimulation of helper T (TH) cells will be provisionally characterized by examination of antibody-selected PR8 virus mutants, or of enzymatic HA cleavage products, for ability to stimulate TH hybridomas. The determinants will then be defined precisely by production and analysis of synthetic peptides. An attempt will be made also to characterize HA-structures involved in the formation of the target antigen(s) recognized by cytotoxic (CTL) cells. This may be achieved by selection of PR8 mutants in the presence of CTL clones and subsequent antigenic and structural characterization of these virus mutants. 2) The function of longterm anti-viral TH clones, specific for HA or internal viral proteins, will be examined with respect to extent of help for anti-viral B cell responses to HA and internal viral proteins. These comparative analyses will be done in vitro and in vivo in an adoptive transfer system. 3) The relevance of the HA (as target structure for regulatory and effector immune mechanisms) in protection against influenza virus infection will be investigated in a murine model system. This will be done by comparing degree of protection and immune response in PR8 immune mice upon challenge with various reassortant viruses that differ from PR8 only with regard to the HA. The longterm goals are 1) to define the role of distinct regions of the HA as target structures for immune effector cells and as nominal antigen structures for regulatory T cells and 2) to assess how precisely changes in various regions of the HA affect the interaction between influenza virus and immune system of the host.